A high voltage supply of this kind is known for example from EP 0 696 134 B1, which discloses a high voltage supply providing an anode voltage and several grid voltages. It comprises a high voltage transformer with a driver stage, which is coupled to the horizontal deflection stage of the picture tube. One of the secondary windings of the high voltage transformer is the high voltage winding, which comprises usually several high voltage diodes, arranged in a diode split configuration, for providing the high voltage. Additional grid voltages are provided by voltage divider chains, coupled either to the output voltage of the high voltage winding or to an intermediate tap of the high voltage winding.
A new kind of picture tube, known as “eagle picture tube”, requires a supplementary mask voltage, which is higher than the anode voltage. Like conventional picture tubes the eagle picture tube has a shadow mask, but with the difference that this one is composed of two planes separated by a few {fraction (1/100)} of mm. By applying a voltage of a few 100 Volts across the two planes this mask is able to focus more electrons through the holes of the mask, therefore reducing the number of electrons landing into the metal of the mask and increasing the light transmission. Because the mask is working at the EHT voltage, over 30 kV, a second voltage source referenced to the anode voltage has to deliver the few 100 Volts necessary to obtain the optimum focusing effect.
Considering the technologies and currents involved in the EHT generation, solutions based on resistor chains or transformer tapping cannot offer today the mandatory level of stability and adjustability requested for an optimum beam focalization. The new focusing mask for this picture tube requires therefore an additional high voltage transformer or another high voltage supply to provide the additional grid voltage.